1. Field
The present embodiments generally relate to liquefied hydrocarbon fluids, and to methods and apparatus for processing such fluids. The present embodiments more particularly relate to the removal of components with low boiling points such as nitrogen and/or helium from a hydrocarbon stream being processed in a natural gas liquefaction plant.
2. Description of the Related Art
Natural gas is an important energy source that is obtained from subterranean wells, however, it sometimes contains impurities such as nitrogen and helium. In such situations, extraction of the impurities, such as nitrogen rejection, can be performed. Helium can also be present in natural gas, and can be separated for further processing in a helium recovery plant.
Raw natural gas contains primarily methane. It also can contain smaller amounts of ethane, propane, n-butane, isobutane, and heavier hydrocarbons, as well as water, nitrogen, helium, mercury, and acid gases such as carbon dioxide, hydrogen sulfide, and mercaptans.
Natural gas can be converted to liquefied natural gas (LNG) by cooling it to about −161° C., depending on its exact composition, which reduces its volume to about 1/600th of its volume at standard conditions. This reduction in volume can make transportation more economical. The liquefied natural gas (LNG) can be transferred to a cryogenic storage tank located on an ocean-going ship. The production of refrigeration needed to liquefy the natural gas is generally one of the highest expenses within a LNG liquefaction plant.
The presence of nitrogen in the LNG can increase the cost of transportation and decrease the heating value of the natural gas. A common solution to nitrogen contamination is the rejection of nitrogen. The stream containing the extracted nitrogen may contain hydrocarbons that may be used for purposes such as blending into a fuel gas stream.
Helium may be present in natural gas and can be recovered as a product. Helium may be separated from the natural gas utilizing methods that produce a helium enriched gas stream that can then be further processed in a helium recovery facility.
In light of the above, it is desirable to have an effective method to reduce the nitrogen concentration of an LNG stream, extract a helium enriched stream from said LNG stream, and reduce the refrigeration needs of the LNG liquefaction plant.